Pectins are a family of complex polysaccharides that contain 1,4-linked
α-D-galactosyluronic acid residues. Pectins account for ~30% of the
primary walls of dicotyledenous and non-graminaceous monocotyledenous plants
and for between 5 and 10% of the walls of grasses. Pectins are also likely
to be present in the walls of ferns, lycopods, and bryophytes. The presence
of pectin in algal walls needs to be substantiated. There is no evidence
that bacteria or fungi produce pectin.

Three pectic polysaccharides have been isolated from plant primary cell walls and structurally characterized. These are:

Homogalacturonan (HG) is a linear chain of 1,4-linked α-D-galactosyluronic acid residues in which some of the carboxyl groups are methyl esterified (see the figure on the right). HG with a high degree of methyl esterification is referred to as "pectin" whereas HG with a low degree of methyl esterification is "pectic acid". HGs may, depending on the plant source also be partially O-acetylated.

HG may account for up to 60% of the pectin in a primary wall. Many of the
properties and biological functions of HG are believed to be determined
by ionic interactions. Increases in the degree of methyl esterification
of HG have been implicated in increased cell separation.

Enzymes that fragment HG (endo and exo polygalacturonases and lyases) are secreted by many plant pathogens. The oligogalacturonide fragments generated have been shown to induce several plant defence responses (see Plant Pathogens).

The sequencing of the Arabidopsis genome has revealed the existence of
at least 52 genes encoding putative polygalacturonases. However, there
is little know about the function or specificities of these enzymes.

Studies with transgenic plants that over-express a specific endopolygalacturonase
(EPG) or have had the levels of endogenous EPG reduced have begun to provide
information about the function of HG. For example, neither the supression
or over-expression of EPG in tomato fruits affected the fruit ripening
process but did alter tissue texture and pectin solubility. In contrast
transgenic apples containing additional copies of a fruit-specific EPG
exhibited premature leaf shedding, reduced cell adhesion, brittle leaves,
and had malformed stomata.

It is likely that HG together with other pectic polysaccharide have numerous
roles in plant growth and development

HG biosynthesis is a major area of research at the CCRC (see Biosynthesis)

Unusual ester cross links in pectin

The existence of esters that form between the carboxyl group of a 4-linked
GalpA residue and a hydroxyl group of another glycosyl residue has been
hypothesized. The presence of non-methyl galacturonosyl esters has been
implied from chemical analysis and FT-IR spectroscopy. The trans-esterification
of pectin has been reported to be catalyzed by pectin methyl esterases.
However, no fragments of HG containining unusual esters have been isolated
and structurally characterized.

Current research

1.The use of chemical and enzymic methods to isolate and characterize non-methyl
esters in pectin.

Xylogalacturonans have β-D-Xylp residues attached to C-3 of the galacturonan backbone (see right figure).
Xyloglacturonans are only present in reproductive tissues including soybean
seed, apple fruit, and pine pollen. Little is known about the function
of this polysaccharide, although it has been shown that the presence of
Xylp residues limits the ability of fungal EPGs to fragment the backbone

Apiogalacturonans are present in the walls of some aquatic monocots including Lemna and Zostera. The backbone is substituted with β-D-Apif and β-D-Apif-(1,3')-β-D-Apif-(1, side chains (see right figure). The backbone of these polysaccharides
is resistant to fragmentation by microbial EPGs.

Current research

1. The biosynthesis of apiose and xylose - further details are available in the biosynthesis page.